Don't use keg-only and pouring jargon.

This is unnecessarily confusing to end-users and the "fun" naming doesn't really communicate anything. @mistydemeo and I talked about this.

CC @adamv @mistydemeo @jacknagel @tdsmith @asparagui for thoughts and review.
I'm not sold on "private" because it has normative connotations from OOP; e.g. a class's private methods either cannot or should not (depending on the paradigm) be accessed by something external to that class. It is not the case that builds outside of Homebrew cannot or should not use a Homebrew OpenSSL.

"Unlinked by default" is correct I don't think we're likely to find a more concise explanation of the concept without inventing more jargon. Avoiding kebab case where we can probably improves readability.